The present invention relates to a liquid crystal display apparatus, and, more particularly, the invention relates to a liquid crystal display apparatus employing a lateral electric field method which is driven by applying an electric field in a direction nearly parallel to the substrate surface.
In prior art liquid crystal display apparatuses, the electrodes which are used for driving a liquid crystal layer are transparent electrodes disposed opposite to each other and respectively formed on two substrates. This structure comes from employing a display method typified by a twisted nematic display method in which the direction of the electric field applied to the liquid crystal is orientated in a direction nearly normal to the substrate surface. On the other hand, a method of employing comb-shaped electrodes, in which the direction of the electric field applied to the liquid crystal is orientated in a direction nearly parallel to the substrate surface, is proposed, for example, in Japanese Patent Publication No. 63-21907, U.S. Pat. No. 4345249, WO 91/10936, Japanese Patent Application Laid-Open No. 6-22239-7 or Japanese Patent Application Laid-Open No. 6-160878. In this case, the electrodes do not need to be transparent, and so highly conductive, opaque metal electrodes are employed. In regard to the above-mentioned prior art display method in which the direction of the electric field applied to the liquid crystal is orientated in a direction nearly parallel to the substrate surface (hereinafter, referred to as the xe2x80x9clateral electric field methodxe2x80x9d), a method of reducing unevenness in the display, such as a domain on the electrode edge existing from the initial stage of use, has been proposed in Japanese Patent Laid-Open No.7159786, but there is no description therein concerning the elimination of unevenness in the display, which is seen as black spots produced during long-term use, or concerning the structure necessary for improving the productivity of the apparatus.
The inventors of the present invention have newly found that when a liquid crystal display apparatus of the lateral electric field type is used continuously for a long time, unevenness in the display, seen as black spots (hereinafter referred to as xe2x80x9cblack stainsxe2x80x9d), is produced. As a result of studying the cause of the black stains, it has been revealed that the black stains are produced by the following process.
(1) The liquid crystal is directly in contact with an electrode through a crack in a protective film.
(2) Electrochemical reaction occurs on the electrode due to a signal voltage applied to the electrode to produce an ionic substance.
(3) The produced ionic substance exudes onto the liquid crystal layer to reduce a voltage holding ratio, and consequently the spot is seen as black.
It can be seen from the above-mentioned considerations that the problem of black stain production can be solved by preventing any direct contact between the liquid crystal layer and the electrode.
An object of the present invention is to provide a liquid crystal display apparatus in which black stains are not produced. Another object of the present invention is to provide a method of manufacturing a liquid crystal display apparatus without producing black stains and with an increased productivity.
According to the present invention, by forming a thick protective film or thin electrodes, cracks do not occur in the protective film, and consequently the occurrence of black stains can be prevented. The cracks in the protective film are formed by cracks in a thin portion of the film due to a stress applied to the film. Particularly, the cracks are apt to occur in edge portions of the electrodes because thickness of the protective film there is thin. The reason for this is as follows.
An SiN film commonly used for the protective film has a characteristic of film growth only in a direction normal to the substrate. Therefore, in the case of a common electrode cross-sectional structure, where the edge portion of the electrode is nearly 90 degrees or a reentrant etch, the edge portion of the electrode cannot be covered with the protective film until the film thickness of the protective film becomes thicker than the film thickness of the electrode. Accordingly, the film thickness of the protective film at the edge portion of the electrode is determined by the difference between the film thickness of the protective film and the film thickness of the electrode film.
Therefore, in order to form a sufficiently thick protective film at the edge portion of the electrode, to prevent occurrence of the cracks, it is necessary that the film thickness of the protective film is sufficiently thicker than the film thickness of the electrode. The inventors of the present invention have concentratively studied the difference between the film thickness of the protective film and the film thickness of the electrode film necessary for eliminating black stain faults, and they have found as a result that the thickness of the protective film should be thicker than the film thickness of the electrode under the protective film by more than 0.4 xcexcm.
Further, according to the present invention, the black stain faults can be eliminated by forming the protective film using an organic polymer film, instead of using an inorganic film formed through a film forming method under vacuum, such as the CVD method. The organic polymer film can be formed through the common wet method using a solution.
The wet film-forming method using a solution is composed of the three processes of (1) forming the solution film over the whole surface, (2) drying the solvent and flowing in the lateral direction of the solution, and (3) forming the film by drying the solvent. Since the film is formed over the whole surface of the substrate by the process (1) and the solution flows from the upper portion of the electrode to the lower portion to make the film thickness uniform by the solution flow in the process (2), a film which is relatively thick can be formed also in the edge portion of the electrode, as compared to the case of an inorganic film. The organic polymer film is so soft that cracks hardly occur, compared to an inorganic film, such as a SiN film.
For the above-mentioned reason, it is possible to reduce the black stain defect by using an organic polymer film for the protective film.
The materials capable of being used for the organic polymer film are various kinds of organic polymer, such as polyimide, acrylic polymer, epoxy polymer and benzocyclobutene polymer.
Further, according to the present invention, it is possible to suppress the occurrence of black stain faults by employing an electrode having a tapered etch. Since film growth takes place, even on an electrode having a tapered etch, from the initial stage, a protective film having a sufficient thickness also can be formed in the edge portion. Therefore, the occurrence of cracks can be suppressed to reduce the black stain faults. An electrode with a tapered etch can be formed by using an appropriate etching gas or an appropriate etching solution. For example, for a chromium electrode, an electrode with a tapered etch can be obtained by using an etching solution of cerium-sulfate-ammonium or the like. For an electrode made of MoTa, an electrode with a tapered etch can be obtained by using a mixed gas of CF4 and O2.
Furthermore, according to the present invention, it is possible to suppress the occurrence of black stain faults and reduce the film thickness of the protective film by employing a sufficiently thick alignment film without pin holes. That is, it is possible to eliminate black stain faults without decreasing the productivity.
Since the alignment film is formed as a uniform film without pin holes when the film thickness of the alignment film becomes a certain value, the alignment film can not only orientate the liquid crystal, but can serve as a protective film. Therefore, by forming the alignment film so that it is thick, it is possible to suppress the occurrence of black stains even if the protective film is thin. The alignment film having a sufficient film thickness can be obtained by a method of increasing the concentration of the solution without increasing the film-forming time. Therefore, increasing the thickness of the alignment film and reducing the thickness of the protective film shortens the film-forming time of the protective film. That is, there is an effect of improving the productivity.
The inventors of the present invention have concentratively studied the film thickness of the alignment film necessary for eliminating pin holes as well as for eliminating black stain faults, and they have found as a result that the alignment film should be thicker than 0.1 xcexcm.